This invention relates to optical fiber cables specially adapted for drop line applications.
Fiber-to-the-premises (FTTP) from local telephone and cable service providers is being implemented at a rapid pace. This service requires a broadband optical fiber distribution network comprising local optical fiber distribution cables that are installed in neighborhood and city streets. The local distribution cable is a large fiber count (multi-fiber) cable. Single fiber or few fiber cables are used for the xe2x80x9cdropxe2x80x9d line from the street to the premises. In many cases, aerial drop lines are used, and these have special requirements.
Optical fiber drop cables are made in several designs. Most of these designs mimic earlier copper cable versions. Physical resemblance is deliberate, so that the external cable appearance matches that of existing copper versions, and standard hardware and installation equipment may be used for both. Thus xe2x80x9cA-dropxe2x80x9d optical fiber cable is an optical fiber version of A-drop copper cable, and is made in the same flat or ribbon-like configuration. Aerial drop cable typically has one or more strength members for support. A common A-drop or flat cable design comprises one or more optical fibers between two strength members. See for example, U.S. Pat. No. 6,501,888.
Optical fiber cables also commonly contain gel-filling compounds for preventing water excursion in the cable. When water enters a cable, flow of water along the length of the cable is blocked by the gel. However, gel filled cables are difficult to install and repair. Moreover, since the drop wire is typically attached to the side of a customer""s home or building, bleeding of ingredients in the cable onto the customers building may cause cosmetic or other problems.
Since aerial drop cables are subjected to considerable movement and sag due to wind and ice build-up, and due to mechanical strain caused by differential thermal expansion, aerial drop cables commonly have a loose fiber design. In this design the optical fibers float within the cable encasement. The premise is that the optical fibers are mechanically isolated from at least some of this movement. However, a drawback to this design is that the cable may suffer fiber retraction due to the movement just mentioned. Fiber retraction occurs when the outer sleeve of the optical fiber cable sags or is stretched relative to the optical fibers. Excessive fiber retraction may result in damage or breakage of the fibers.
Several examples of drop cable are described in U.S. Pat. No. 4,761,053. Most of these examples describe copper drop wire but a few optical fiber versions are given as well. These show loose fiber designs as just mentioned.
An optical fiber cable design that resembles an A-drop design, i.e. has a relatively small, flat, cross section, is described in Japanese Patent JP-A-8304675. In this design the strength members are optical fibers. Since the optical fiber strength members require separate coatings, this structure adds unnecessary elements, complexity, and cost.
We have designed an optical fiber cable suitable for drop cable applications which has an xe2x80x9cA-dropxe2x80x9d design, meaning it is readily interchangeable with existing copper cable drop-wire technology. The new A-drop optical fiber design is dry, and has a primary encasement that is coupled directly to the optical fiber(s). We discovered that, contrary to conventional practice, increasing the coupling between the optical fibers and the exterior surface of the cable provides unexpected benefits, and reduces the tendency of optical fiber cables to buckle. The coupled design forms a unitary assembly that is more robust, and not susceptible, as are conventional designs, to fiber retraction. This property is especially important in drop cable, and will be described in more detail below. The use of a solid primary encasement, in contrast to the loose fiber designs, or the gel filled designs, allows the desired coupling between the outside cable sheath and the optical fibers. For convenience in this description, the encasement next to the optical fiber(s) will be referred to as the primary encasement, and the polymer coating for the overall cable assembly will be referred to as the secondary encasement. In the main embodiments of the invention the primary encasement is conformal to the optical fiber assembly and the secondary encasement is conformal to the primary encasement.
The optical fiber drop cable design of the invention comprises three in-line components encased in the secondary encasement. The center component of the cable contains the optical fiber(s) coupled to the primary encasement. The optical fiber(s) may comprise a single fiber, two or more bundled fibers, or one or more optical fiber ribbons, each with two or more fibers ribboned together. In each case, the optical fiber or optical fiber assembly (two or more fibers) is coated with the primary encasement. Reference to optical fiber assembly below is intended to be generic to all of these arrangements. Reference to optical fiber system below, refers to the center component of the cable, i.e. the optical fiber assembly of one or more fibers, and the primary encasement that is coupled to the optical fiber assembly.
Located on each side of the optical fiber system are strength members. Typically, these comprise high strength polymer or glass strands in a resin matrix. Dimensions of the three elements are chosen so that the diameter of the optical fiber system is less than the nominal diameter of either strength member. This provides crush protection for the optical fiber assembly.
In an alternative embodiment, the cable is round on the end portions, with the middle portion, the portion that contains the optical fiber system, dished so that the hyphen between the end portions has concave sides.
Increased coupling of the optical fiber assembly to the primary encasement, and the primary encasement to the secondary encasement, to yield the advantages mentioned above, is achieved in part by adhesion between both the optical fibers and the primary encasement and the optical fiber system (primary encasement) and the secondary encasement. Relatively high adhesion between the optical fiber assembly and the surrounding medium is important to prevent retraction of the optical fiber assembly within the cable.